The invention relates to semiconductor wafer testing.
Mobile ion contaminants within a silicon dioxide (SiO2) oxide layer disposed over a silicon semiconductor wafer can cause problems in the manufacture and performance of integrated circuits. For example, copper metal deposited on the oxide surface to enhance circuit performance can penetrate into the oxide layer during high temperature annealing (400xc2x0 C. to 500xc2x0 C.) processes used during IC manufacturing. During prolong circuit operation, copper present in the oxide reduces the oxide layer""s resistivity, which increases leakage through the oxide layer and degrades overall circuit performance. Other ionic impurities such as Na+, Li+, and K+ also can be introduced within the oxide layer during high temperature processing. The most common technique for measuring the total concentration of mobile ions in the oxide layer include the capacitance-voltage (CV) method and the triangular voltage sweep (TVS) method. These methods are described in D. K. Schroder, Semiconductor Material and Device Characterization, John Wiley and Sons, Inc. (1990), pp. 263-267, hereby incorporated by reference.
Generally, these methods include preparing metal oxide semiconductors (MOS) capacitor patterns on test wafers and applying a voltage to a metal while heating the wafer to move the ions. In the CV methods, the total mobile ion concentration is determined from the differences in capacitance-voltage characteristics caused by a drift of ions; and specifically by the shift in so-called xe2x80x9cflat band voltagexe2x80x9d. In the TVS method, the total mobile ion concentration is determined from the electric current component due to mobile ion drift across the oxide.
This invention relates to a non-contact, non-destructive method to determine different mobile ion concentrations directly from a change in the contact potential value caused by differential mobile ion redistribution toward or away from the top of the oxide surface.
In one aspect, the invention features a method of measuring at least two different ion concentrations within an oxide layer of a semiconductor. The method includes applying a first predetermined BTS-conditioning to the semiconductor wafer including the oxide layer disposed thereon to cause ions of a first type to migrate within the oxide layer; and applying a second predetermined BTS-conditioning to the semiconductor wafer to cause ions of a second type to migrate within the oxide layer. The first predetermined BTS-conditioning does not substantially cause the ions of the second type to migrate within the oxide layer.
Embodiments of this aspect may include one or more of the following features. The method further includes measuring the first contact potential before and after the ions of the first type migrate within the oxide layer and measuring the second contact potential before and after the ions of the second type migrate within the oxide layer. The method includes a pre-BTS conditioning to cause a random distribution of ions of the first and second types to move into a non-random distribution such as migrating to the surface of the oxide layer or the Si/SiO2 interface. The method includes measuring the oxide leakage current at the first and the second predetermined BTS-conditionings, correcting the first contact potential with the oxide leakage current measured at the first predetermined BTS-conditioning, and correcting the second contact potential with the oxide leakage current measured at the second predetermined BTS-conditioning.
The first and second predetermined BTS-conditionings each include biasing the semiconductor wafer with a predetermined charge from a corona charging element and heating the semiconductor wafer to a predetermined temperature for a predetermined time period. The first and second charges, temperatures, and time durations can be the same or different. The first and the second temperatures each are between about 150xc2x0 C. and about 300xc2x0 C.; the first and the second charges each are between about 0.1 to about 6 MV/cm; and the first and the second time periods each are between 30 sec and 3600 sec. The first predetermined BTS-conditioning includes biasing the semiconductor with a charge of about 0.5 MV/cm and heating the semiconductor to about 170xc2x0 C. for a time period of at least about 2 minutes for a 1000 xc3x85 oxide thickness. The second predetermined BTS-conditioning includes biasing the semiconductor with a charge of about 1.5 MV/cm and heating the semiconductor to about 170xc2x0 C. for a time period of at least about 20 minutes or biasing the semiconductor with a charge of about 1.5 MV/cm and heating the semiconductor to about 225xc2x0 C. for a time period of at least about 3.5 minutes for a 1000 xc3x85 oxide thickness. The ions of the first type, such as Na+, have an ion mobility that is larger than the ion mobility of the ions of the second type, such as Cu+, at a constant temperature. The semiconductor wafer can include a metal layer, such as copper, periodically patterned on a top surface of the oxide layer.
In another aspect, the invention features a method for determining different mobile ion concentrations within an oxide layer disposed on a surface of a semiconductor wafer including depositing a first charge on at least a portion of the surface of the oxide layer at a low temperature at which a first mobile ion does not substantially move, measuring the contact potential on the surface of the oxide layer, heating the semiconductor wafer and oxide layer to a first temperature sufficient to force substantially all of the first mobile ions to migrate across the oxide layer, measuring a first shift in contact potential after said heating to the first temperature, determining the first mobile ion concentration within the oxide layer on the basis of the first shift; depositing a second charge on at least a portion of the surface of the oxide layer at a low temperature at which a second mobile ion does not substantially move, measuring the contact potential on the surface of the oxide layer, heating the semiconductor wafer and oxide layer to a second temperature sufficient to force substantially all of the second mobile ions to migrate across the oxide layer, measuring a second shift in contact potential after said heating to the second temperature, and determining the second mobile ion concentration within the oxide layer on the basis of the second shift. The method can further include determining the oxide leakage current at the first charge and the second charge using the oxide leakage at the first charge and the second charge to determine the first and second ion concentrations.
In another aspect, the invention features a system for the measurement of mobile contaminant ion concentration in an oxide layer of a semiconductor wafer. The system includes a charge deposition device configured to deposit charge on the oxide layer of the wafer; a temperature stress device including a element for heating the wafer to a temperature sufficient to allow mobile ions to drift; a measurement device configured to measure the contact potential; and a semiconductor wafer holder including at least one semiconducting wafer having an oxide layer disposed on a surface of a semiconductor wafer. The oxide layer includes a metal layer patterned onto its surface.
Embodiments may include one or more of the following advantages. The system provides a fast, accurate, and reliable technique for measuring the concentration of different types of mobile ions within the oxide layer of a semiconductor wafer. The technique is non-destructive and during the entire cycle the wafer is contacted only from the back side for the purpose of holding, moving, heating, and cooling the wafer. Thus, the wafer can be characterized without having to sacrifice a portion of the wafer. Moreover, because the technique can be performed relatively quickly, the concentration of different mobile ions can be mapped on the entire region (with the exception of the uncharged reference region) rather than in only particular points on the wafer. The technique makes possible the scanning or mapping of the different mobile ion distribution over the entire wafer surface in a realistic time, e.g., about 10 to 30 minutes for an 8 inch diameter size wafer.
Further features, aspects, and advantages, follow.